In conventional inductive ignition systems for internal combustion engines, spark plug discharge current is typically characterized by an initial high current peak followed by a subsequent current decay. An example of such a conventional discharge current waveform 150 is illustrated in FIG. 8. One known problem associated with the operation of conventional spark plugs is electrode erosion caused by excessive discharge currents and/or excessive discharge current duration. In either or both cases, electrode erosion leads to poor or at least reduced engine operational efficiency and, eventually, spark plug failure.
Another class of ignition systems include specially configured spark plugs which are operable to propel the arc away therefrom to facilitate combustion of lean air-fuel mixtures and reduce electrode erosion rate. One example of such a spark plug includes a magnet positioned about the electrodes, wherein the magnetic field is operable to propel the arc outwardly from the plug. One embodiment of such a spark plug is described in U.S. Pat. Nos. 5,555,862 and 5,619,959 to Tozzi, which are assigned to the assignee of the present invention, and the disclosures of which are incorporated herein by reference. With such spark plugs of this nature, two key goals are to maximize the ability to ignite fuel at lean air-fuel mixtures while also maximizing electrode life. Unfortunately, the conventional discharge current waveform 150 illustrated in FIG. 8 is not optimized to further either of these goals. Excessive discharge current too early in the ignition event results in excessive electrode erosion while inadequate discharge current near the end of the ignition event results in poor combustion.
What is therefore needed in either a conventional or an arc-propelling spark plug based ignition system is a technique for controlling spark plug discharge current magnitude and/or duration to thereby maximize electrode life, and in an arc-propelling spark plug based system a technique for further controlling spark plug discharge current near the end of an ignition event to thereby maximize the ability to ignite fuel and lean air-fuel mixtures.